This invention relates to an anode member for a solid electrolytic capacitor and, in particular, to an anode member including a valve metal thin plate as an anode lead and a sintered layer formed on a surface of the thin plate, a solid electrolytic capacitor having the anode member, and a method of producing the anode member.
An electrolytic capacitor comprises a metal acting as an anode and a dielectric oxide film formed on a surface thereof by anodic oxidation. The dielectric oxide film is brought into contact with an electrolytic solution or a solid electrolyte acting as an opposite electrode to accumulate electric charges between the anode and the opposite electrode. Because the electrolytic capacitor is small in size and large in capacity, the electrolytic capacitor is already put into practical use and research and development are continuously made for the purpose of further improvement in characteristics. As the metal to be subjected to anodic oxidation, a so-called valve metal is used.
For example, in a solid electrolytic capacitor using the valve metal, such as tantalum (Ta) or niobium (Nb), the valve metal for the anode is often used in the form of a sintered product having a porous structure so that the anode has a large specific surface area and, consequently, the capacitor has a large capacitance. In order to obtain the sintered product, a powder of the valve metal is press-formed into a desired shape and then sintered in a vacuum at a high temperature.
Generally, the sintered product having a circular cylindrical or a rectangular cylindrical shape is used in the solid electrolytic capacitor. For convenience of description, the solid electrolytic capacitor of the type may be called a cylindrical solid electrolytic capacitor. On the other hand, Japanese Patent Application Publication No. S59-219923 (JP 59-219923 A) discloses a solid electrolytic capacitor comprising a valve metal thin plate or foil and a valve metal sintered layer formed on a surface of the thin plate. For convenience of description, the capacitor of the type will be called a foil-type solid electrolytic capacitor.
In the foil-type electrolytic capacitor disclosed in the above-mentioned publication, an anode member is formed by preparing the valve metal thin plate as an anode lead, applying a valve metal powder onto the surface of the anode lead to a desired thickness, and then sintering the valve metal powder into the valve metal sintered layer.
Hereinafter, the foil-type solid electrolytic capacitor disclosed in the above-mentioned publication will be described in conjunction with an Nb solid electrolytic capacitor by way of example. It is well known in the art that, in the foil-type solid electrolytic capacitor, the sintered product of the valve metal electrically serves as the anode. In the following description, the valve metal thin plate and the valve metal sintered layer formed on the surface thereof may collectively be called an anode member.
For example, the foil-type solid electrolytic capacitor using Nb comprises an Nb foil as the valve metal thin plate, an Nb powder sintered layer as the valve metal sintered layer, an Nb oxide thin film, a solid electrolyte layer, an external anode terminal, an external cathode terminal, and a resin package. A combination of the Nb foil and the Nb powder sintered layer forms an Nb foil anode member.
In the foil-type solid electrolytic capacitor, the Nb powder sintered layer is formed on the surface of the Nb foil. The Nb powder sintered layer has a porous structure with microscopic holes formed inside and therefore has a very large specific surface area. On an outer surface of the Nb powder sintered layer and on a surface of an inner wall of each microscopic hole formed inside, the Nb oxide thin film is formed by anodic oxidation. The Nb oxide thin film serves as a dielectric member of the capacitor.
On a surface of the Nb oxide thin film, the solid electrolyte layer is formed. The solid electrolyte layer serves as a cathode of the capacitor. A combination of the Nb powder sintered layer as the anode, the Nb oxide film as the dielectric member, and the solid electrolyte layer as the cathode forms a fundamental structure of the capacitor.
On a surface of the solid electrolyte layer, a conductive substance layer is formed and called a cathode conductor layer. The cathode conductor layer comprises, for example, a graphite layer and a silver paste layer successively deposited. A combination of the solid electrolyte layer and the cathode conductor layer may be called a cathode layer. To the outermost layer of the cathode conductor layer, the external cathode terminal is fixedly attached for electrical connection with the outside. On the other hand, the Nb foil has an exposed part where the Nb powder sintered layer is not formed thereon. The external anode terminal for electrical connection with the outside is fixedly attached to the exposed part.
Furthermore, the resin package comprising epoxy resin or the like covers the Nb foil, the Nb powder sintered layer, a part of the external cathode terminal, and a part of the external anode terminal. Another part of each of the external cathode terminal and the external anode terminal which is not covered with the resin package is shaped along the resin package, i.e., folded downward along a side surface of the resin package and further folded inward onto a bottom surface of the resin package.
In the foil-type solid electrolytic capacitor having the above-mentioned structure, the Nb foil serves to electrically connect the Nb powder sintered layer as the anode of the capacitor and the external anode terminal. Thus, the Nb foil corresponds to a metal wire which is embedded in the cylindrical sintered product in the cylindrical solid electrolytic capacitor and generally called the anode lead.
The foil-type solid electrolytic capacitor mentioned above is advantageous in reduction in size and thickness of the capacitor because the anode member is easily reduced in thickness as compared with the cylindrical solid electrolytic capacitor. Since the contact area between the Nb foil as the anode lead and the Nb powder sintered layer as the anode is increased so that the resistance therebetween is decreased, it is possible to reduce ESR (Equivalent Series Resistance) of the capacitor.
As compared with the cylindrical solid electrolytic capacitor, the foil-type solid electrolytic capacitor has the above-mentioned advantages. However, the foil-type solid electrolytic capacitor is disadvantageous in that production is difficult as compared with the cylindrical solid electrolytic capacitor in the reasons which will hereinafter be described.
The anode member of the foil-type solid electrolytic capacitor is obtained by preparing the Nb foil as the valve metal thin plate, depositing an Nb powder layer on the surface of the Nb foil, for example, by applying a paste with Nb powder particles dispersed therein, and sintering the Nb powder layer.
Generally, the sinterability of a metal powder, i.e., the degree of cohesion or fusion of the powder particles forming the powder, or the degree of growth of the powder particles is widely different depending upon the situation. Specifically, the sinterability at a boundary between the particles within the Nb powder layer is widely different from the sinterability at an interface between the particles and a metal object such as a metal foil or a metal thin plate. It is known that, even at the same temperature, sintering is quickly promoted between the powder particles while the growth or the fusion of the particles is difficult or slow between the powder particles and the metal foil or the like.
It is assumed that, in the anode member of the Nb foil-type electrolytic capacitor, a relatively low sintering temperature is selected, focusing upon the porosity of the Nb powder sintered layer. In this event, an excellent sintered condition is obtained in a region inside the Nb powder sintered layer. On the other hand, at the interface between the Nb foil and the Nb powder sintered layer, sintering is insufficient so that the fusion or the cohesion between the Nb foil and the Nb powder sintered layer is weak. This results in frequent occurrence of defective products due to separation or release of the Nb powder sintered layer from the Nb foil during handing in a production process. In addition, the capacitor is deteriorated in leakage current characteristic.
In order to avoid the trouble during the production process and the deterioration in characteristic of the capacitor as a result of the relatively low sintering temperature, the sintering temperature is elevated so as to promote and enhance the fusion or the cohesion at the interface between the Nb foil and the Nb powder sintered layer. In this event, however, the Nb powder sintered layer is over-sintered and significantly deformed due to shrinkage following the progress of sintering, resulting in occurrence of separation and cracks. In case of such significant deformation, the dimensional accuracy of a half product or a final product is difficult to satisfy. In addition, in various steps in the production process, mechanical stress is often caused to occur at portions deformed in the sintering. This results in an increase in frequency of occurrence of destruction of the Nb oxide film and a degradation in leakage current characteristic of the capacitor. In most cases, cracked portions are associated with much significant deformation so that the above-mentioned troubles are more likely to occur.
Thus, for the anode member of the foil-type solid electrolytic capacitor, it is necessary to accurately control the sintering temperature in the sintering step to an appropriate sintering temperature. However, depending upon the variation in powder particle size and particle size distribution per each material lot, the appropriate sintering temperature is different to determine. Therefore, it is extremely difficult to accurately control the sintering temperature in the production process. In addition, the deformation and the cracks mentioned above are more prominent as the thickness of the powder sintered layer is increased. Therefore, the thickness of the powder sintered layer is restricted so that the capacitance of the capacitor can not be increased.
On the other hand, the anode member of the cylindrical solid electrolytic capacitor does not use the foil but uses the metal wire as the anode lead. The anode lead comprising the metal wire is surrounded by the Nb powder. When the Nb powder is press-formed into a cylindrical shape, the anode lead and the Nb powder are press bonded to each other. Thus, at a stage prior to sintering, the cohesion between the anode lead and the Nb powder is already strong. Even if sintering is carried out at a relatively low temperature so as to assure the porosity, the cohesion between the anode lead and the powder sintered product after sintering is sufficiently strong as compared with anode member of the foil-type solid electrolytic capacitor. In addition, the press-forming has an effect of uniformizing the density of powder particles within a press-formed product so that the deformation is minimized even if the sintering temperature is elevated.
As described above, the anode member of the foil-type solid electrolytic capacitor has production difficulty in the process of sintering.